This invention relates to a method for treating alkali metal halide brines to stabilize silica containing colloidal complexes therein, when the treated brines are used in membrane electrolytic cells.
Alkali metal halide brines for use in membrane electrolytic cells are concentrated solutions which are prepared by dissolving the alkali metal halide in water or a less concentrated aqueous brine solution. The impurities in the brine produced vary in both types and concentration with the source of salt. Typically, the brine which is a neutral solution, contains as impurities significant concentrations of calcium, magnesium, iron, and silica as well as lower concentrations of complex-forming elements such as aluminum, zinc, tin, and lead.
To remove impurities such as calcium, magnesium and iron the brines have traditionally been treated with basic salts such as alkali metal carbonates and alkai metal hydroxides to produce as insoluble precipitates the carbonates and hydroxides of these elements. These precipitates are removed by well known settling or filtering methods. During these treatment and separation steps the concentration of silica is also reduced along with that of other elements in ionic form which react with the brine treatment agents to produce insoluble compounds.
As the ion exchange membranes employed in membrane cells are easily damaged by even moderate concentrations of elements such as calcium and magnesium, the alkaline brine is further purified by methods such as ion exchange processes.
Such brines typically have not only a pH of between about 4 and about 12, a calcium content of between about 20 and about 60 ppb, and correspondingly low contents of iron, magnesium, sulfate, chlorate and carbonate ions, but also an aluminum content of between about 0.1 and about 2.5 ppm and a silica content of between about 0.1 and about 20 ppm.
Prior to feeding the highly purified concentrated brine to the electrolytic membrane cells, the brine is acidified by the addition of an acid such as hydrochloric acid to reduce the pH to less than 4, for example, about 2-3.
During electrolysis of these brines in electrolytic membrane cells, a certain amount of hydrochloric acid and hypochlorous acid form in the brine. Even though these acids may be partially neutralized by backmigrating hydroxyl ions coming from the catholyte compartment, their concentration increases, so the anolyte pH remains highly acidic.
Maintaining the pH of the brine at highly acidic levels during electrolysis produces a chlorine gas of high purity as well as maximizing the operating efficiency of the membrane by neutralizing hydroxyl ions which backmigrate through the membrane.
The production of alkali metal halide brines as neutral solutions and the subsequent treatment under alkaline conditions stabilizes any complex containing complex-forming elements such as aluminum and silica where present in the salt or brine source. In addition, the use of mineral products such as perlite or diatomaceous earths as filter aids in the filtering or ettling methods results in increasing concentrations of these elements as well as silica in the brine.
While not wishing to be bound by theory, it is believed that in alkali metal chloride brines, the silica forms a hydrophobic colloidal sol which is readily peptized by the negative chloride ions in the brine so as to be quite stable and difficult to coagulate. Where positive ions are also present, they are strongly attracted by the negatively charged colloid to form colloidal particles of a metal silica complex which are small in size, non-aggregatable and non-ionic. Thus, they are not readily removable by precipitation, filtration or ion exchange treatments, such as those described above used to produce "conventional" membrane cell quality brines.
Where the brine is acidified before being fed to the electrolytic cell or in many cell systems using high performance membranes of a type which effectively suppress such backmigration, such as the carboxylate/sulfonate composite described in U.S. Pat. No. 4,202,743, issued May 13, 1980 to Oda et al., during cell operation the pH of the anolyte solution is maintained within a range of about 2 to about 3. However, at such a pH, it is found that many of these complexes dissociate with any complex-forming elements present being converted to the positive ionic form. In a membrane cell, these positive ions are transported, during electrolysis, into the membrane wherein on contact with the strongly basic catholyte solution, they tend to precipitate therein, and this results in a permanent loss of membrane efficiency.